The present invention relates to ballast and ignition electronics for high intensity discharge lamps, particularly in automotive headlamps.
High intensity discharge (HID) lamps generate light via an electric arc between electrodes in a sealed transparent tube filled with a gas and containing elements that determine the light spectrum. To start the arc, the gas must be ionized. This can be done by a high voltage initial pulse between the electrodes provided by ignition electronics. Once the gas is ionized, and the other elements are heated and ionized, an electrically conductive plasma exists in the tube, reducing the gap impedance. The voltage is then controlled by ballast electronics to maintain the arc for maximum lamp efficiency and life. The arc and electronics produce heat that can accumulate in a lamp enclosure and damage parts in the enclosure, including the lamp electronics and the enclosure itself. This is especially problematic in automotive headlamps, since they must be sealed in an enclosure for protection from contaminants in the slipstream air. It has been difficult to provide an HID headlamp and electronics as a single unit due to this enclosed heat accumulation. Conventionally, the lamp electronics are kept outside the enclosure to avoid damage from heat inside the enclosure, and to avoid contributing to such heat.
Compact fluorescent bulbs contain ignition and ballast electronics in their base. Although fluorescent lamps use an electric arc, they are not considered HID lamps. Their lower intensity does not produce high heat, so they do not encounter this degree of heat problem.
U.S. Pat. No. 6,710,545 (Yamaguchi et al.) shows an arc discharge lamp 30 with a directly coupled electronic controller 40. However, this system is enclosed in a casing 11 with no external radiator for the electronics, allowing heat to build-up in the casing 11. Also, the lamp/controller unit is mounted with only one point 20a of support, and does not contact the casing 11. This allows the lamp to vibrate. Vibration in HID automotive headlamps is a problem. It can damage the lamp or loosen its connections. It can also cause loss of alignment and/or an apparent flicker, which are dangerous distractions to oncoming drivers.